1. Field of the Invention
The present invention relates to metal finishing and in particular to the provision of a durable, wear resistant finish on aluminum alloy castings which is suitable as a bearing surface.
2. Description of the Prior Art
Aluminum and aluminum alloys are popular materials for fabrication of light weight articles. The fabrication of such articles can involve machining from wrought aluminum or casting from aluminum alloy. Casting is considerably less expensive than machining. However, casting is generally unsuccessful when attempted with pure aluminum. Instead, specially formulated aluminum alloys are employed which allow the molten material to flow during the casting process. These specially formulated alloys often contain substantial concentrations of silicon.
For wear resistant articles, aluminum is most commonly finished by anodizing, an electro-chemical process in which electrolytically generated oxygen chemically combines with surface aluminum to form a hard, durable aluminum oxide surface layer. Unfortunately, the silicon containing alloys most suitable for casting do not form acceptable anodized finishes. The silicon component of the alloys interferes with or interrupts the formation of a continuous and strongly adhering aluminum oxide layer on the surface of the casting material. The resulting anodized coating is insufficient to provide a hard, durable, wear resistant surface.
Aluminum castings including a durable, hard bearing surface with a reduced tendency to generate particulates under bearing use and wear conditions would be particularly useful in close tolerance, high precision mechanical environments which must remain substantially dust free. Disk drive systems are an example of such an environment. Particulate contamination within a disk drive tends to cause the flying head to crash into the rapidly spinning disk, thereby damaging or destroying both magnetically stored data and the disk drive itself.
Presently, aluminum based castings requiring hard, durable bearing surfaces, are produced by the addition of a separate piece of non-cast, pure aluminum to the casting. Subsequent to the addition, the article is anodized. Thus, the economic advantage of casting, (specifically, the avoidance of expensive machining operations) is lost with the addition of the pure aluminum.
The problem of providing a casting with an anodized finish which allows a uniformly dyed appearance is addressed by Furukawa in U.S. Pat. No. 4,444,628. The disclosed process comprises the steps of chemically polishing a casting, barrelling and/or blasting the chemically polished casting to mechanically treat the surface and remove irregularities, followed by degreasing, anodizing, dyeing and sealing.